How to Design Discrete Opto-Coupler Circuit for 2+A·sin(2πft) Input, 2A Output, LED Indication

Design Specifications

Students are required to design a circuit where the output is isolated from the input by an opto-coupler that is constructed from discrete components (to be supplied), i.e. an infrared light emitting diode (LED) and a phototransistor

The input signal to the circuit is specified as 2 + A sin(2π f t) Volts where 20Hz< f < 20kHz and 1 < A < 4 Volts. Note that this will be the signal that is applied to test the operation of the circuit during the project demonstration.

The design specifications are:

1. The output signal is to be 2A sin(2π f t + θ) where f is the frequency and A is the amplitude of the input signal applied to the circuit.

2. The phase shift, θ, with respect to the input signal, must be as small as possible.

3. The circuit will detect when the specified input signal is applied and indicate this by switching on a green LED.

4. The circuit is to switch on a red LED when the frequency, f , of the output (or input) signal is between 970Hz and 1.03 kHz. 5. The circuit is required to minimise the effects of nonlinearities, due to the ‘optocoupler’, as observed in the output signal.

Each group of students will be provided with,

• an infrared LED,

• a phototransistor,

• a breadboard,

• access to operational amplifiers and ancillary components (e.g. resistors, capacitors, diodes and LED’s),

• a model of the infrared LED and photo-diode for use in LTspice

1. The obvious thing to do, is to build a voltage-to-current converter to drive the LED, and a current-to-voltage converter driven by the phototransistor. The LED-current to phototransistor-current transfer function is linear, though its gain will vary with spacing, optical path and lenses, and temperature. The voltage-to-current converter will require a DC offset, to shift from bipolar input to unipolar output. The current-to-voltage converter will require a DC offset, to shift from unipolar input to bipolar output.
Since the signal to be passed from input to output is AC only, AC coupling using series capacitors can be used.

2. To minimize high-frequency phase shift, the circuit bandwidth must be wide compared to the signal bandwidth. If AC coupling is used, phase shift must be checked at low frequencies also.

3. An opamp used as a comparator can do this.

4. Look in the ARRL Handbook, particularly decades-old editions of The Radio Amateur's Handbook and old issues of QST, for audio filters for receiving CW (continuous-wave) morse-code radio signals.

4. If "ancillary components" includes a filament transformer (120V 60 Hz to 12V or 6V), the transformer's primary winding can be used as an inductor in an audio tuned circuit.

4. If "ancillary components" include CMOS logic gates, multiplexers, and flipflops, an N-path analog filter can be built to give high frequency selectivity.

Extra-credit option: Build a first-order one-bit deltasigma analog-to-digital converter to drive the LED, and a one-bit first-order deltasigma digital-to-analog converter driven by the phototransistor.

This Project involves using LM741 Op Amps

could you elaborate how i would do this